1. Field of Invention
This invention relates to a method and apparatus for firing a series of print hammers associated with a moveable member, like a rotating drum, for example.
2. Background Information
FIG. 1 shows the geometry of a drum printer of the prior art. A rotating drum carrying a plurality of characters around the periphery thereof is mounted for rotation in a frame of a printer. Several hammer assemblies (extending into the plane of the drawing) are mounted in a hammer frame and are arranged to impact against a ring of characters positioned around the periphery of the drum. An ink ribbon and the document to be printed upon are positioned between the characters on the rotating drum and an associated hammer tip for each one of the hammer assemblies. Each hammer assembly includes a hammer frame, coil, backstop adjustment screw, and hammer tip which are assembled in the general arrangement shown in FIG. 1. When current is applied to the coil, the associated tip is propelled forward to strike the associated character on the rotating drum.
The time taken by the hammer tip to travel from its rest position to the strike position is termed the flight time. Usually, the printer is adjusted so that the desired hammer is fired one "flight time" ahead of the character to be printed. To accomplish this, a drum position sensor is positioned at a physical angle to lead the characters on the drum by one flight time.
The flight time for any one hammer is dependent upon several variables including:
1) Distance from the rest position to the drum.
2) Magnitude of the drive current in the associated energizing coil.
3) Magnetic field strength around the hammer being fired.
4) Strength of the return spring.